Fiber optic gyroscopes are used for detection of rotation particularly in navigation systems such as those used in aircraft and spacecraft. Fiber optic gyroscopes are desirable due to the high-level of accuracy and reliability of sensing inertial rotation rate that they possess.
Fiber optic gyroscope technology is well known in the industry. In a fiber optic gyroscope, light from a laser or some other light source is divided into two separate beams by means of a fiber optic coupler and then coupled into the two ends of a multi-turn coil of optical fiber. The fiber optic cable may consist of many of the standard types commercially available on the market. Light that emerges from the two fiber ends is combined by the fiber optic coupler and detected by a photodetector.
The fiber optic gyroscope senses rotation rate by detection of a rotationally induced phase shift between the light beams that propagate in opposite directions around the coil of the fiber optic cable. The signal that is detected corresponding to the phase difference between the counter-propagating beams is typically subjected to some form of phase modulation. The photodetector converts the modulated light beam to an electrical signal that corresponds to the rate of rotation of the coil of optical fiber. The signal is processed to provide a direct indication of the exact rate of rotation of the coil of optical fiber that has occurred.
Other physical phenomena may contribute to phase differences between the counter-propagating light beams than the mere physical rotation of the fiber optic gyroscope. Some of the most common performance limiting phenomena include: micro-bends in the fiber within the wound coil; polarization cross-coupling of the light within the coils; and most notably, the inconsistencies due to the winding process of the coil.
Losses due to micro-bends or any non-orthogonal relationship, between symmetrical points in the fiber optic coil and the center axis of the fiber optic coil are exacerbated by external temperature variations. When a thermal gradient passes through a fiber optic coil, the change in temperature produces a change in the refractive index of the material from which the fiber is made. An asymmetrical change in the refractive index of the fiber will cause a phase shift between the clockwise and counterclockwise paths of the rotating beams of light passing through the coil. Cross-coupling of the polarization states within the coil may also cause unwanted phase shifts.
Furthermore, when the winding process of the coil is more difficult, the machines used for winding the coil are more difficult to automate and maintain the precision winding pattern without excessively stressing the fiber. A poorly wound coil leads to the loss and polarization cross-coupling discussed above. If these external phenomena are introduced into the gyroscope, the unwanted phase shifts or losses cause the fiber optic gyroscope to indicate false measurements that translate into false readings of rotation rate.
The largest contributor to these losses and phase shifts occur in the region of the coil where each turn makes a transition to the next turn or wind. This section of each wrap is known as the "Jog Zone". In the past, the length of this zone was kept to a minimum. Unfortunately, a short jog zone will maximize these performance-limiting errors in high-performance fiber optic gyroscopes. In order to reduce the errors and sensitivities discussed above, it would be desirable to provide a new jog zone configuration that is independent of the coil configuration, i.e. quadrupole, octupole, interleave, or other variations thereof.
In the prior art, the length or angle of the jog zone was intentionally kept to a minimum in order to maximize the orthogonal winding section of the coil. U.S. Pat. No. 4,793,708 issued to Bednarz entitled "Fiber Optic Sensing Coil" teaches a symmetric fiber optic sensing coil for use in a rotation rate sensing device such as a gyroscope. This patent teaches that the jog zone of the fiber optic coil should consist of only 5 to 10% of the entire winding of the coil. Although this patent discloses that the jog zone is an essential portion of the entire winding of the coil, it does not suggest any basis for choosing the 5 to 10% figures. Furthermore, the patent does not teach that inherent advantages would exist by increasing the angle of the jog zone to a size that minimizes losses due to micro-bends, that when combined with external temperature fluctuations, can create unwanted phase shifts. The present application proposes the improved jog zone size that is entirely independent of the coil configuration.